Probably because there are other people looking for the same issue they're having before posting their issue. Something you should have done with that handy search bar prior to posting yours.
It's a known issue that sometimes the launcher corrupts the saved data in your profile. Re-enter it and it should work correctly.
You may need to reset your password with mojang. Make sure you're using your e-mail and not your username to log in.
I think I covered all of the usual suspect with login issues.